Theoretically, the use of a hybrid coil matching such a carrier system to a bidirectional two-wire local line should completely isolate the two trunk branches from each other as far as voice signals are concerned. In practice, however, the hybrid coil is subjected to a minor unbalance or mismatch giving rise to echoes that return to the remote caller with a certain time lag dependent on the physical length of the trunk (which could, in part, be constituted by radio links).
Such echoes become particularly annoying when the time lag exceeds 50 ms. Various means have therefore already been proposed for dealing with the problem. One known solution utilizes an echo suppressor which, upon detecting an incoming voice signal, inserts a high attenuation in the echo path. That attenuation, of course, must be removed as soon as the other subscriber begins to talk; the unavoidable delay in such removal results in chopped or mutilated words. In the case of "double speech", i.e. with both subscribers talking simultaneously (as is especially frequent with long-range communication via satellite), the echo suppressor is entirely ineffectual.
An alternative solution resides in the use of a so-called echo canceler, e.g. as described in an article by J. A. Webb and M. W. Kelly, entitled "Telephone Echo Cancellation For Satellite Terminals", ICC 78, Paper No. 10.5.1. See also an article entitled "Analysis of an Adaptive Impulse Response Echo Canceller" by S. J. Campanella, H. G. Suyderhoud and M. Onufry, published in COMSAT Technical Review, Vol. 2, No. 1 Spring 1972, and another article entitled "An Experimental Adaptive Echo Canceller For Long Distance Telephone Circuits" by H. Brueggemann, G. Kidd, L. K. MacKechnie and A. J. Seyler, published in The Radio And Electronic Engineer, Vol. 43, No. 4, April 1973. Such an echo canceler detects voice signals on the receiving branch of the trunk line and calculates, on the basis of circuit parameters such as the pulse response of the hybrid coil, the resulting echoes expected to appear on the transmitting branch. This echo replica is subtracted from the effective echo signals, thus reducing subscribers' annoyance. The known echo cancelers operate digitally and in some instances use recursive transversal filtering for progressive reduction of the estimate error. The estimate may also be obtained, according to the above-identified article by J. A. Webb and M. W. Kelly, by constructing an artificial model of the echo channel through test signals which may consist of a pseudorandom signal sequence.
Adaptive echo cancelers, continuously adjusting themselves to changes in the response characteristics of the echo channel, require complex circuitry operating with high-speed algorithms. Even so, the adapting mechanism must be deactivated during double speech.
One of the factors affecting the echoes, especially in a system utilizing frequency-division multiplexing, is the phenomenon known as "phase roll", i.e. the phase shift occurring in the echo channel. This phase roll, resulting from noncoherent demodulation of the FDM carriers at points of the unidirectional line branches remote from the points of modulation, causes an offset of all voice-frequency components between the transmitted speech signals and their returning echoes. Such a frequency offset, though limited to not more than 3 Hz by CCITT recommendations, exerts a significant influence upon the estimate elaborated by the echo canceler.